Superconducting switches for various electrical circuit applications have been studied, and several different devices have been developed. Some superconducting switches, such as the cryotron developed by Dudley Buck in the 1950's and the superconducting flux-flow transistor (SFFT) developed in more recent years, rely on an externally-applied magnetic field to quench superconductivity between current-carrying terminals of the device. (See e.g., U.S. Pat. No. 2,832,897; G. K. Hohenwarter, “Superconducting High TC Thin Film Vortex-Flow Transistor,” Sponsored Research Report (1991) available at www.dtic.mil/dtic/tr/fulltext/u2/a235025.pdf.) Because these devices switched a large region of the device between a “superconducting” state and a “normal” state, the devices tend to exhibit slow switching speeds, e.g., on the order of 10's of microseconds.
In the late 1960's, alternative approaches to fabricating superconducting switches incorporated Josephson junctions into a multi-layer integrated device. (See e.g., J. Matisoo, “The Tunneling Cryotron—A Superconducting Logic Element Based on Electron Tunneling,” Proc. IEEE, Vol. 55, No. 2, February 1967, p. 172.) The Josephson junction devices include a tunnel junction at a gate region of the device. The junction can be switched between a superconducting state and a normal tunneling state by the application of a current pulse that exceeds a critical current level at the Josephson junction. Because the junction volume is small, the device can be switched quickly, e.g., at switching times of the order of 1 picosecond. However, Josephson-junction-based devices have a number of limitations including: (1) sensitivity to magnetic fields, (2) limited gain, (3) inability to drive large impedances, and (4) difficulty in controlling the junction critical current (which depends sensitively on sub-Angstrom-scale thickness variation of the tunneling barrier). Additionally, fabrication of the device requires multi-layer and multi-material processing, as well as precise control of the tunneling barrier's thickness.
Recently, superconducting sensors and amplifiers have been developed for applications such as single-photon detection. (See, O. Quaranta et al., “Superconducting Three-Terminal Amplifier/Discriminator,” IEEE Trans. Appl. Supercond., Vol. 19, No. 3 (2009) p. 367.)